Originally, all photographs were developed in a darkroom; this term carries historical significance. Old photo studios had their own darkrooms.
Pro Tip 2/3
Darkness Required
The room must be dark to prevent light from exposing photographic materials and ruining the photos. She closed the door tightly to keep the darkroom lightproof.
Pro Tip 3/3
Red Safe Light
Darkrooms often use a red light called a safe light, as it doesn't affect photographic paper. The red glow in the darkroom came from the safe light.